High-power fiber lasers and particularly amplifiers with output powers of tens or hundreds of watts, sometimes even several kilowatts are becoming more in demand. Lasers with high output powers are required for a number of applications, e.g. for material processing (welding, cutting, drilling, soldering, marking, surface modification); large-scale laser displays; remote sensing (e.g. with a LIDAR); medical applications (e.g. surgery); military applications (e.g. anti-missile weapons); and fundamental science (e.g. particle acceleration), among others.
A typical high-power laser requires one or several powerful pump sources. Pumping with high-power laser diodes has become more widespread. High power lasers are often configured as a Master Oscillator/Power Amplifier (MOPA), where a low power, high beam quality master oscillator is amplified to high output power in one or more power amplifiers. A high power adaptive optics (AO) is typically placed after the power amplifier and used to correct any phase distortions imparted on the beam during amplification. However, at multi-kilowatt power levels, creating a suitably powerful master oscillator and an adaptive optics with suitable power handling is problematic.
Injection seeders are devices that direct the output of small seed lasers into the cavity of a much larger laser to stabilize the larger laser's output. Most seed lasers are stable, single-frequency lasers that emit within the linewidth of the larger laser's gain medium. The single frequency of the seed laser encourages the larger laser to emit in a single longitudinal mode. Seed lasers can be continuous or pulsed. Seeding a pulsed laser can reduce variations in the output energy and timing (jitter) from pulse to pulse, and smooth out temporal variations within the pulse. Many commercial lasers use a laser diode as a seeding source. However, an extra laser for a seeding source, adds to the cost and complexity of the laser device.
High power laser systems are being considered for variety of industrial applications, e.g., cutting, drilling, welding, and heat treating, and military directed energy weapon applications with respect to a variety of platforms, e.g., spaceborne, airborne and land based systems to name a few. To achieve application objectives, high power laser systems must be accurately steered and optimally focused. Steering involves line-of-sight control and focusing, with respect to HEL systems, involves wavefront error correction. Currently, wavefront error correction is typically achieved using adaptive optics. In addition, one or more fast steering mirrors may be used to correct for tilt and direct the line-of-sight. A plurality of wavefront sensors are typically employed along with an aperture sharing element (ASE).
However, scaling solid state HELs is a complicated task and typically results in degradation of the output beam due to waste heat deposited in the lasing medium and the attendant thermal lensing and birefringence. That is, the higher the power of the laser, the higher the distortion of the laser output beam. Existing solutions for multi-kilowatt, near diffraction limited, solid-state lasers generally fall into one of three categories: 1) Power oscillators with intra-cavity high power adaptive optics; 2) single beamline MOPA systems with one or more amplifiers in series; and 3) beam-combined systems with multiple parallel smaller power oscillators or MOPA's. The first category, shown in FIG. 1, is normally used with low gain laser materials.
FIG. 1 is a simplified block diagram of a power oscillator high gain amplifier, according to prior art. For a typical solid state laser, the laser gain medium is optically pumped with laser diodes and achieves a population inversion with a single-pass gain that exceeds unity. Laser signal originates from optical noise in the cavity and is amplified via stimulated emission as it oscillates between the highly reflecting mirror and the partially reflecting mirror due to constructive feedback. The deformable mirror located within the laser cavity adjusts the wavefront (or phasefront) of the oscillating laser beam such that the phase is flat at the wavefront sensor's aperture. Re-imaging optics (not shown) are typically inserted in the path of the sample beam that is transmitted through the fold mirror (or beamsplitter) which reimage the entrance aperture of the wavefront sensor to the aperture of the deformable mirror, thereby ensuring accurate correction. An adaptive optics (AO) controller receives the electrical signal from the wavefront sensor and generates actuator commands that drive the deformable mirror to a shape that is the conjugate (or reverse) of the sensed wavefront. The result is a higher quality output beam with minimal residual wavefront distortion. One limitation of this power oscillator (PO) architecture is that the intra-cavity intensity is larger than the output beam intensity by the inverse of the partially-reflecting mirror reflectivity which for low-gain media can be very high. This subjects the laser gain medium to high thermal stress and exacerbates the beam degradation due to thermal lensing and birefringence. It also subjects the deformable mirror element to the same high intra-cavity intensity, which may exceed the power handling limitations of these devices for some applications.
The second category, shown in FIG. 2, requires a relatively high power Master Oscillator (MO) (with powers typically ranging from 1% to 10% of the system output power) in order to keep the amplifier gain reasonably low for efficient operation and to minimize the deleterious effects of optical feedback into the MO. These lasers are also typically implemented with high power adaptive optics at the output but could potentially use an adaptive optics located between the MO and the power amplifier (PA), where the power on the deformable mirror is lower.
The architecture of the MOPA in FIG. 2 differs from that of the power oscillator of FIG. 1, because there are no mirrors defining a laser cavity surrounding the high power gain medium (Power Amplifier) and the signal originates from a separate external master oscillator that may be fiber coupled into the power amplifier with an isolator stage that minimizes feedback from the amplifier into the MO. A beam shaping lens is included at the output of the fiber to expand and shape the beam for efficient coupling into the power amplifier aperture.
A third category laser can reduce the power loading of the amplifier elements and the power incident on the adaptive optics by 1/N where N is the number of parallel beams combined, however, the complexity level of such systems leads to high cost, reduced reliability, and large size/weight. A sensor at the output (not shown) measures the phase (piston error) between the output beams which drives the average phase of each DM.